EXAMPLES OF SENSITIZATION BY CHLOROPHYLL 507 



C. Chlorophyll as a Sensitizer in vitro* 

 1. Examples of Sensitization by Chlorophyll 



In the preceding discussion, we mentioned several mechanisms of 

 chlorophyll bleaching in which this process appeared merely as a side 

 reaction associated with the sensitized oxidation of the solvent, added 

 "acceptors," or impurities. We shall now discuss these sensitized 

 reactions in more detail. The distinction between a "photochemical 

 reaction of a dyestuff" and a "reaction sensitized by a dyestuff" often 

 is merely one of emphasis. Authors who are interested in the transfor- 

 mations of the pigment often do not care much about concomittant 

 changes suffered by other components of the reacting system; while 

 investigators whose interests are centered on the effects of light on the 

 "substrates" of sensitization often ask no questions as to the fate of the 

 sensitizing pigment. It was suggested on page 56 that the term "photo- 

 catalysis" be used for reactions in which the sensitizing pigment is 

 known to remain unchanged. We shall have to deal here with some 

 truly photocatalytic reactions, but also with some whose photocatalytic 

 character is by no means certain. 



Our knowledge of the sensitizing properties of dyestuffs, including 

 chlorophyll, developed from two sources. In 1874, Vogel discovered the 

 sensitization of the photographic plate, and Becquerel found that chloro- 

 phyll can be used for this purpose. A quarter of a century later, Raab 

 discovered that Paramoecia are killed by visible light in the presence of 

 certain dyestuffs, and thus initiated the study of the " photodynamic 

 effects" (a term introduced by von Tappeiner and Jodlbauer in 1907). 

 Hausmann (1908) and Hausmann and Kolmer (1908) found that extracts 

 from green plants are " photodynamically active," and Hausmann (1909) 

 made a similar observation with pure chlorophyll solutions. Basically, 

 "photodynamic effect" is the same phenomenon as Vogel's and Bec- 

 querel's "sensitization." Von Tappeiner and Jodlbauer called their book 

 Sensititizing Effects of Fluorescent Dyestuffs, and suggested the term 

 "photodynamic effect" only as a provisional one, to be used for "bio- 

 logical" sensitizations until their nature was better known. However, 

 this term became widely accepted and is often used by biologists even 

 when dealing with reactions in vitro, such as the autoxidation of iodide 

 in the presence of eosine (c/. Spealman and Blum 1937). 



Certain authors (c/. Gicklhorn 1914) believed in fundamental differences between 

 photodynamic effect and ordinary sensitization. They quoted, for example, the 

 necessity of molecular oxygen for photodynamic action (a photographic plate can 

 be sensitized in nitrogen), or the fact that only fluorescent dyes' are photodynamically 

 active (whereas nonfluorescent dyes can be used in photography). However, the first 



* BibUography, page 524. 



